The present invention concerns neutron logging techniques and, more specifically, a method and apparatus for correcting a detected spectrum of gamma radiation from the distorting effects of gamma radiation produced by the capture of thermal neutrons in a borehole.
It is well known that the presence of oil or gas in an earth formation can be detected by examining the spectrum of gamma radiation received from the formation after it has been irradiated with neutrons. The interactions between the neutrons and the formation nuclei by which the gamma radiation is produced are also well known. Commonly, such gamma ray spectra are obtained by passing a neutron source through a borehole and measuring the intensity of the resultant gamma radiation as a function of borehole depth.
When using such logging techniques, and particularly when it is desired to monitor the production of capture gamma radiation from the formation, significant errors are caused by capture gamma rays produced in the borehole. Such borehole capture gamma radiation is formed by scattered neutrons that decay to thermal energy levels and reenter the borehole to be captured by the borehole fluid, by the pressure housing and by the borehole casing if such is present. The problem is particularly acute with hydrogen, in which case the borehole signal is usually larger than the signal from the earth formation.